The overarching goal of this study is to explore the context of rising drug use along the Mexico/Guatemala border and define its relationship to the local epidemiology and phylo-geography of HIV and sexually transmitted infections (STIs). Shifting drug trafficking routes, poor economic conditions, and increasing popularity of hard drugs among youth may be some of the factors driving substance use. Numerous regional and international migrants passing through the area likely contribute to the spread of HIV and STIs. However, data is lacking on the relationship between drug use and HIV/STIs in the region. Based on the above, the specific aims of this project are to: 1) describe the contextual factors affecting drug use and patterns of use in high risk populations along the Mexico/Guatemala border; 2) determine the prevalence and correlates of HIV, hepatitis C virus (HCV), and STIs among substance users; and 3) explore the phylo-geography and molecular epidemiology of HIV-1 infection in at-risk groups. To meet Aim 1, 20 in-depth interviews with a diverse sample of substance users in Tapachula/Ciudad Hidalgo, Mexico and 20 in the Teczn Uman/Quetzaltenango area of Guatemala will be conducted to provide contextual data on risk behaviors and help refine study instruments and methods. Additionally, focus groups with substance using men who have sex with men (MSM), female sex workers (FSW), and migrants, as well as with organizations serving those at-risk for HIV will be conducted. To meet Aim 2, three consecutive cross-sectional cohorts (n=400, 200 on each side of the border) will be recruited using respondent-driven sampling (RDS). Face-to-face interviews will collect information on sociodemographics, HIV/STI knowledge, risk behaviors, and access to health services. All will be tested for HIV, syphilis, herpes simplex virus-2, and HCV, and treated as needed. Round 1 will comprise substance using FSWs; Round 2 will focus on the MSM population; and Round 3 will begin with peer recruiters (seeds) drawn from mobile persons- including truck drivers, deportees and regional and international migrants. We anticipate and will be able to measure the overlap between these risk groups. For Aim 3, we will sequence the HIV-1 pol gene, including reverse transcriptase, from those infected with HIV which will enable us to assess if recombinants or drug resistant strains of HIV-1 have emerged in this region and the extent to which there is mixing between at-risk groups. As migration routes have been identified as important corridors of disease transmission in other settings, they may be one of the first places new HIV subtypes will be identified. This project will create and expand ties among investigators in Mexico, Guatemala, and the U.S. and increase regional research capacity. Findings will help inform the development of subsequent HIV interventions and prevention programs that intervene upon risky substance use behaviors before they become further entrenched. Since the HIV epidemic in most of Latin America is still concentrated, there is a critical window of opportunity to prevent transition to a generalized epidemic.